006

temperatures up to 1200°C in various industries. This alloy, known as alloy 602CA (UNS N06025), employs the beneficial effects of high chromium, high aluminum, high carbon, and microalloying with titanium, zirconium, and yttrium. Developed in the early 1990s, the alloy has found numerous applications in various industries as mentioned above. The typical chemical composition of the alloy in weight percent is given below:

Ni Cr Fe Al C Ti Zr Y Bal 25 9.5 2.2 0.18 0.15 0.06 0.08

This alloy is covered in ASTM and other international specifications. ASME code case 2359 has been approved for SC VIII, Div. 1, and SC I (steam service only) up to 1650°F. AWS coverage for weld filler metal in A5.11 and A5.14 is under progress. The major properties of interest in this alloy are:

• Excellent oxidation resistance up to 1200° C, superior to other wrought nickel base alloys currently available in the market

• Good high-temperature strength (stress rupture and stress to produce 1% creep at temperatures up to 1200° C), superior to most other Ni base alloys over 1000° C

• Excellent carburization resistance

• Excellent metal dusting resistance

Alloy 602CA employs the beneficial effects of high chromium, high aluminum, high carbon, and microalloying with titanium, zirconium, and yttrium in

FIGURE 7.4 Microstructure of annealed alloy 602CA (x500).

a nickel matrix. The relatively high carbon content of approximately 0.18-0.2% in conjunction with 25% chromium ensures the precipitation of bulky homogeneously distributed carbides, typically 5-10 in size. Transmission and scanning electron microscopy suggest these bulky carbides to be of M23C6-type primary precipitates. Microalloying with titanium and zirconium allows the formation of finely distributed carbides and carbonitrides (Fig. 7.4). Solution annealing even up to 1230°C does not lead to complete dissolution of these stable carbides, and thus the alloy resists grain growth and maintains relatively high creep strength due to a combination of solid solution hardening and carbide strengthening. This phenomenon of grain growth resistance is responsible for maintaining good ductility, a high creep strength up to 1200°C, and superior low-cycle fatigue strength. Table 7.12A shows the grain growth data for various high-temperature alloys where alloy 602CA had very little grain growth even after approximately 1000 h of exposure at 2050°F (1121°C). Hence repair and reconditioning of exposed parts can easily be achieved with alloy 602CA. The presence of approximately 2.2% aluminum in this alloy allows the formation of a continuous homogenous self-repairing Al2O3 sublayer beneath the Cr2O3 layer, which synergistically imparts excellent oxidation as well as carburiza-tion and metal dusting resistance: "Reactive elements" like yttrium significantly increase the adhesion and spallation resistance of the oxide layers, thereby further enhancing the high-temperature corrosion-resistant properties. Also, because

TABLE 7.12A Effects of High-Temperature Exposure on Grain Growth for Various Alloys Exposed at 2050°F (1121°C)

Average ASTM Grain Size Number

TABLE 7.12A Effects of High-Temperature Exposure on Grain Growth for Various Alloys Exposed at 2050°F (1121°C)